The present invention relates to the field of thermal history indicators and time-temperature indicators. These are devices which display a physical change in response to their temperature history and are typically attached to or integrated with temperature sensitive goods in order to provide a quality control and/or quality assurance indicator.
Many goods sold at the present time are temperature sensitive. For example, fresh food produce needs to be kept in a rigidly temperature controlled environment until it is sold. This has implications for manufacturers, distributors, retailers and consumers.
Distributors are faced with the technological problem of maintaining temperature of goods within a very tight specification for local, national and international distribution of goods. As a result of this need, it is necessary to verify that goods have been distributed under the required conditions for reasons of quality control and quality assurance.
Manufacturers and retailers have a duty of care to their customers. When dealing with produce that is temperature sensitive, they must not only control and verify the temperature under which goods are stored and processed internally but will also want to receive proof that raw materials and supplies have been looked after properly. When mistakes in temperature regulation are not noted, goods may be spoiled and unsellable or, worse, may lead to damage to the consumer for which the vendor becomes liable.
Consumers also face problems related to the temperature control of goods they have purchased. To take an example, milk can spoil extremely quickly if allowed to warm up for a period of time. Consumers would benefit from a way of finding out whether or not retailers are storing goods appropriately. Furthermore, consumers would prefer to purchase goods which they believe have been stored correctly prior to their purchase.
At the present time, businesses and retailers typically use thermometers and thermocouples to monitor temperature throughout the food chain. Consumers will not usually monitor temperature of their purchases.
Several organisations currently manufacture and sell time-temperature indicators. These are devices which can be attached to, or be incorporated in packaging and which provide a visual indicator of the temperature history of the label and, therefore, the produce to which it is attached.
The 3M Monitor Mark contains a dye which moves along a scale when the indicator is above a certain melting point. This suffers from the disadvantages of not being edible, not having a clear link between the length the dye has moved along the scale and the temperature history of the product and also requires to be kept below its freezing point before use.
The Lifelines Fresh-Check Indicator uses time and temperature sensitive polymers which gradually deepen in colour. The product is considered to be off when an inner ring made of temperature sensitive material becomes darker than an outer ring. This suffers from the disadvantage that the range of thermal sensitivities which the polymer can adopt are not continuous. Usually, different sensitivities are achieved by varying the colour of the printed outer ring. Care is also required by the user when deciding whether the inner or outer ring is darker. This product also requires refrigeration before use and, indeed, must be kept at a particularly cold temperature to ensure that the sensor has not been triggered. Examples of relevant Patents are U.S. Pat. No. 5,709,472; U.S. Pat. No. 4,892,677 and U.S. Pat. No. 4,735,745.
VITSAB sell time temperature indicators in which an enzyme reaction causes a solution to change from deep green to bright yellow as a result of a controlled pH decrease. A reference colour is printed nearby to enable a viewer to establish whether temperature storage conditions have been violated. One useful benefit of this technology is that the two solutions involved are separated by a divider which can be manually broken, mixing the two solutions (See SE508602 and WO9838112). This allows the label to be transported at ambient temperature and to be activated only when it is ready for use. However, this tag is expensive and fragile and may leak; it is not clear whether the enzyme and chemicals involved are entirely non-toxic. Furthermore, as the colour changes gradually, it becomes difficult for a user to judge when the colour has reached the shade of the reference colour.
These products have so far not been commonly used due to their expense, supply problems with raw materials, limitations to their applicability, toxicity, fragility, sensitivity and the difficulty of manufacture.